The present invention relates to methods and apparatus for obtaining samples of fluids present in a well, and more particularly to the obtaining of samples representative of said fluids at a given depth of the well.
The fluids present in a well are generally in the form of a complex mixture of oil, gas and water; in order to study the properties of this mixture at a given depth, it is desirable to take a sample of the fluid under real conditions of volume, pressure, and temperature, and to keep at least one of these dimensions constant until more analysis can be effected. A simple solution is to keep the volume of the sample constant and then to reheat it to the temperature at which the sample was taken .
U.S. Pat. No. 3,095,930 proposes such a solution, which consists in sampling the fluid while allowing a practically zero pressure drop on admission to a sampling vessel. Once the sampling vessel is full, a mechanical control automatically closes the fluid inlet orifice without overcompressing the fluid. The sample, when it has been raised to the surface, is transferred at constant pressure and volume into a transportation vessel for taking it to a laboratory for analysis.
The solution of this prior patent has certain drawbacks. Firstly the transfer operation performed at the surface of the well appears to be dangerous since it is performed at very high pressures which, in particular, require mercury equipment. Further, since only one sample can be taken per descent, and since the sample must be transferred to a transportation vessel on arrival at the surface, a considerable amount of equipment such as the drilling or production tower tends to stand idle for non-negligible lengths of time. Finally, a control rod inside the sampling vessel must be used to close the vessel, which substantially reduces the volume available for the sample.
Fluid samplers have already been described, in particular in the above-mentioned U.S. Pat. No. 3,095,930 and in French Pat. No. 71 02 766 published under the No. 2,123,178. The complex nature of the fluids to be sampled is explained in these documents, as is the polyphase character of their flow. Complete analysis of these fluids can only be performed in a laboratory. It is thus necessary to be able to reproduce in the laboratory the conditions under which the fluid was found when it was sampled.